This invention pertains generally to target tracking radars, and, in particular, to a method for countering the effects of a stand-off barrage-type jammer on such radars.
As is known, when a nonfluctuating target is illuminated by a monopulse radar, the variance of the indicated azimuth or angle estimate (obtained by forming the real part of the complex monopulse ratio) is inversely proportional to the ratio of sum channel signal power to difference channel noise power, where noise is internally generated thermal receiver noise. When a barrage jammer is present, a source of noise external to the radar system is effective to bias, i.e. exert a so-called "pulling effect," on the monopulse ratio because noise in the difference and sum channels will be correlated. In consequence, then, when an aircraft target being tracked crosses a region covered by a barrage-type stand-off jammer, the target tracking radar may coast through the jammed region (by extrapolating the target angle measurements obtained before the pulling effect of the jammer is experienced). When such a target emerges from the region covered by a barrage jammer, reacquisition of tracking may be effected by employing an angle gate wider than the angle gate normally employed in the track mode. The process is subject to error because: (a) extrapolation over relatively long time intervals leads to large prediction errors caused by noise; and (b) large dynamic errors are encountered if the extrapolated trajectory of the target does not coincide with the actual trajectory flown.